More and more electronic devices are being designed for low power consumption. Such electronic devices operate and communicate, typically wirelessly, while maintaining low power consumption. For example, low energy devices, such as sensors, may use Bluetooth Smart, which is a standard that uses frequency shift keying (FSK) and Gaussian FSK (GFSK), for wireless communication.
The amplifiers of the low power devices, therefore, should also be low power. Power amplifiers are classified based on the characteristics of the output stage. In particular, the classes are based on the proportion of each input cycle in which the output device passes current. Conventional class-A amplifiers are simpler than class-B and class-AB amplifiers and use a single amplifying transistor that is biased such that the amplifying transistor is always conducting. For differential class-A amplifiers, the biasing point is selected to be equal to the maximum output current to allow for the amplification of the full range of the input signal. Therefore, conventional class-A amplifiers have high quiescent current.
Conventional class-B amplifiers use two amplifying transistors, each operating for half a cycle, in a push-pull configuration. Because of the non-overlapping of the signals of each amplifying device, the class-B amplifier typically has high crossover distortion.
Conventional class-AB amplifiers have a push-pull configuration that operates for more than half a cycle. To operate, class-AB amplifiers use biasing circuits that are typically more complex than the biasing circuits of class-A or class-B amplifiers. The overlap helps reduce the cross-over distortion present in class-B amplifiers at the expense of higher quiescent current. Conventional class-AB amplifiers have lower quiescence current than class-A amplifiers.